WASHINGTON, July 21 (AFP) - President Joe Biden said on Thursday (Jul 21) he was "doing great" after testing positive for COVID-19 at the age of 79, vowing to continue to work while isolating at the White House with only mild symptoms.

Biden's personal physician Kevin O'Connor wrote in an official note that the US president was experiencing fatigue, a runny nose and an occasional dry cough, starting on Wednesday evening.

In a series of posts crafted to show he remains at the country's helm, Biden tweeted a picture of himself working at his desk saying "I am doing great", and a short video clip filmed on a White House balcony.

"Hey folks, I guess you heard this morning I tested positive for COVID," said the president, wearing a blazer, no tie and the hint of a smile.

"Symptoms are mild, I really appreciate all your inquiries and concern ... it's going to be OK."

Announcing the diagnosis, the White House stressed that the president was fully vaccinated and twice boosted. It said Biden was taking Pfizer's Paxlovid pill, an antiviral used to minimise the severity of COVID-19.

"Consistent with CDC (Centers for Disease Control and Prevention) guidelines, he will isolate at the White House and will continue to carry out all of his duties fully during that time," it added.

The president tested positive in a rapid antigen test Thursday morning before the result was confirmed by a PCR (polymerase chain reaction) test.

He is reported to be in good general health, but as the oldest US president ever elected his age heightens concern over the impact of COVID-19.

A physical exam in November last year found Biden "fit for duty".

But facing a punishing workload, Biden - like other presidents before him - is showing the strain: his walk is stiffer, his hair thinner, and his speech less fluent than when he took office.

Politically Biden is also in a tough phrase of his presidency, facing November midterm elections that are forecast to be painful for his Democratic Party, as well as declining personal approval ratings.